1. Field of the Invention
The present invention relates to a stationary induction apparatus, such as a transformer or a reactor, provided with an improved magnetic shield on the inner surface of a tank.
2. Description of the Related Art
Generally, leakage flux from a winding included in a stationary induction apparatus, such as a transformer or a reactor, increases as the capacity of the stationary induction apparatus increases. If leakage flux penetrates a structure, such as a tank wall or a core clamping structure, loss increases, efficiency decrease or local overheating occurs.
A known method of suppressing loss and preventing local overheating installs a highly conductive, nonmagnetic shield, such as a copper or aluminum shield, on the inner surface of the tank wall and induces an eddy current that cancels out leakage flux that penetrates the tank wall in the nonmagnetic shield. Another method of preventing the increase of loss and local overheating places a magnetic shield, i.e., a silicon steel plate having a high magnetic permeability, on the inner surface of the tank wall to absorb leakage flux and to prevent the penetration of leakage flux through the tank wall. The method using the magnetic shield is applied prevalently to large-capacity stationary induction apparatuses.
The stationary induction apparatus has a winding, high-voltage leads leading out from the winding and connected to external bushings, and low-voltage leads leading out from the winding and connected to external bushings. The high-voltage leads are extended through through holes formed in a tank wall into a leader pocket. Since the through holes are formed in the tank wall facing the winding, the magnetic shield disposed in a region including the through holes must be divided into upper and lower parts along a line corresponding to the through holes.
Consequently, the magnetic resistance of a portion of the wall not covered with the magnetic shield increases and leakage flux from the winding penetrates the portion of the tank wall around the through holes. Thus, loss increases, local overheating occurs and satisfactory shielding effect cannot be achieved. The low-voltage leads placed on a side opposite a side on which the high-voltage leads are placed are extended along the inner surface of the tank wall at a position dislocated laterally from a position opposite the winding. However, leakage fluxes created by a high current that flows through the low-voltage leads penetrate the wall through gaps between the plurality of magnetic shields to-cause increase in loss and local overheating.
A structure disclosed in Japanese Patent Laid-open No. Sho 61-219122 is capable of reducing loss that may be produced in the tank wall by the leakage fluxes from the windings and the leads and preventing local overheating. This prior art structure has elongate magnetic shields formed by laminating thin magnetic plates and arranged in an upright position in a lateral arrangement on the inner surface of a tank wall facing windings, and electromagnetic shields of highly conducting plates attached to a tank wall facing leads through which a high current flows. Leakage flux from the winding is absorbed by the magnetic shields, and leakage flux from the leads is repulsed by the reactive effect of eddy currents induced in the electromagnetic shield by magnetic fields created by the current flowing through the leads to prevent the penetration of the leakage flux through the tank wall.
The structure disclosed in Japanese Patent Laid-open No. Sho 61-219122 has the elongate magnetic shields arranged on the inner surface of the tank wall facing the windings, and the highly-conducting electromagnetic shields attached to the tank wall facing leads, absorbs the leakage flux from the winding by the magnetic shields, and prevents the penetration of the leakage fluxes from the leads through the tank wall facing the leads by the reactive effect of eddy currents induced in the electromagnetic shields to reduce loss that may be produced in the wall of the tank.
This prior art structure is intended for application to single-phase transformers and its effect is not necessarily satisfactory with three-phase transformers. In a three-phase transformer having three windings linearly arranged in a tank and leads leading out from the windings, particularly, the low-voltage leads, disposed between the windings, it is possible that both the leakage fluxes from the windings and the leakage fluxes from the leads penetrate the tank wall. Nothing about such a problem is taken into consideration by Japanese Patent Laid-open No. Sho 61-219122 and the prior art structure is unable to reduce loss that may be produced in the walls of the leader pockets into which the leads are extended and the tank cover.
This prior art still has problems to be solved concerning the reduction of loss and the prevention of local overheating in portions of the tank facing the high-voltage leads and the low-voltage leads.
The present invention has been made in view of the foregoing problems and it is therefore an object of the present invention to provide a highly reliable stationary induction apparatus capable of preventing the penetration of leakage flux from windings and leads through tank walls and of preventing the increase of loss and local overheating.
With the foregoing object in view, the present invention provides a means for creating magnetic flux of a polarity opposite that of leakage flux from windings and low-voltage leads by an eddy current induced by the leakage flux on the inner surface of a tank wall having portions facing the low-voltage leads or provides a means for creating magnetic flux of a polarity opposite that of leakage flux from windings and low-voltage leads by an eddy current induced by the leakage flux on the inner surface of a tank wall having portions facing the low-voltage leads and a means for absorbing the leakage flux from the windings on a tank wall facing the low-voltage leads, in which the means for creating the magnetic flux of a polarity opposite that of the leakage flux from the leads is disposed on the tank wall having at least a portion facing the low-voltage leads.
More concretely, a composite shield formed by combining a nonmagnetic shield and a magnetic shield is disposed on the inner surface of a tank wall facing the low-voltage leads, the nonmagnetic shield of the composite shield has a portion facing the low-voltage leads, and a portion of the nonmagnetic shield lies between the windings.
With such a construction, the leakage flux from the windings and the low-voltage leads is unable to penetrate the tank wall, so that loss can be reduced and local overheating can be prevented.